As a subcutaneous indwelling type glucose sensor, one in which an enzyme reagent layer is provided on a working electrode and the enzyme reagent layer (or the whole sensor in general) is covered with an outer layer film for restricting diffusion of a substrate in the enzyme reagent layer is known.
The outer layer film of such a glucose sensor is degraded during use of the sensor (over time elapsing after subcutaneous indwelling). When the outer layer film is degraded, an error occurs in a measurement result of a glucose concentration, and therefore, a technique for detecting that the outer layer film has been degraded (a defect has been caused in the outer layer film) has been developed.
Specifically, a technique in which a voltage for obtaining a response derived from a substrate is continuously applied to a sensor, a time T at which change of a response current value caused immediately after the voltage application becomes smaller or larger than a change threshold value is measured, and it is determined that a defect has been caused in an outer layer film if the time T is smaller than a time threshold value (see, for example, Patent Document 1) has been developed.
[Patent document 1] Japanese Patent Laid-Open No. 2012-176229
According to the aforementioned technique, although it may be detected that the outer layer film of the sensor has been degraded, it is impossible to detect that the enzyme reagent layer of the sensor has been degraded. Besides, the enzyme reagent layer of the sensor may be degraded prior to the outer layer film in some cases. Therefore, there is a demand for a technique in which a state (a normal state, a degraded state or the like) of an enzyme reagent layer of a sensor including an outer layer film and the enzyme reagent layer, such as the aforementioned glucose sensor, may be detected.
Besides, in some of glucose sensors not of the subcutaneous indwelling type, the diffusion is restricted without providing an outer layer film (for example, as in a self monitoring blood glucose sensor for measuring a Cottrell current), but a technique in which degradation of an enzyme reagent layer of such a sensor has also not been developed at present.
Accordingly, an object of an embodiment of the present invention is to provide a technique in which a state of an enzyme reagent layer of a sensor including an outer layer film and a sensor not including an outer layer film may be detected.